Retroreflective sheeting in which a graphic image or other mark is built into the sheeting has found a number of important uses, particularly as distinctive labels useful to authenticate an article or document. For example, retroreflective sheetings in which legends are printed into the sheeting, as taught in U.S. Pat. Nos. 3,154,872; 3,801,183; 4,082,426; and 4,099,838, have found use as validation stickers for vehicle license plates and as security films for driver's licenses, government documents, phonograph records, tape cassettes, and the like.
Galanos, U.S. Pat. No. 4,200,875, teaches a different form of imaged retroreflective sheeting, particularly a "high-gain retroreflective sheeting of the exposed-lens type" in which images are formed by laser irradiation of the sheeting through a mask or pattern. The noted "high-gain" sheeting comprises a plurality of transparent glass microspheres partially embedded in a binder layer and partially exposed above the binder layer, with a metal reflective layer coated on the embedded surface of each of the plurality of microspheres, and with the binder layer containing carbon black to minimize any stray light that impinges on the sheeting. The patent teaches that images formed in such sheeting by laser irradiation can be viewed if, and only if, the sheeting is viewed from the same angle at which the laser irradiation was directed at the sheeting.
The Galanos patent does not explain how an image is formed in the described retroreflective sheeting other than to state that the image is formed by "structural alterations (i.e., modifications)" of the sheeting. Based on our work using conditions assumed to be like those used by Galanos on high-gain sheeting, a likely explanation for the image formation observed by Galanos is a modification of the glass microspheres, e.g., by a localized devitrification, melting, or erosion of the glass at the back edges of the microspheres where the laser beam is focused.
Some advantages of the imaged sheeting taught in the Galanos patent are that the images are seen within sharply defined angular ranges, which could be used as an identifying characteristic to help assure that the sheeting is authentic; and the images can be formed in an already manufactured retroreflective sheeting, thereby avoiding the need for inventories of specially printed retroreflective sheeting and avoiding the need for long process runs to distribute the costs in setting up a printing operation.
However, the imaged sheeting taught in the Galanos patent also has important disadvantages which limit its utility. For example, it provides only black and white images. Also, the imaged sheeting is not useful to provide retroreflective images in outdoor environments where it may receive precipitation, because such precipitation can alter the optical relationships in the sheeting and obliterate or greatly reduce a retroreflective image.